Over the course of the last decade there has been a substantial increase in the number of electrical devices in residential and commercial facilities that utilize switched-mode power supplies. The switched-mode power supplies associated with these non-linear loads typically have rated output powers ranging from a few watts to more than 1 Â kW. Historically, low-cost power supplies incorporated a simple full-wave rectifier that conducted only when the mainline's instantaneous voltage exceeded the voltage on the input capacitors. This resulted in very high ratios of peak-to-average input current, which also resulted in a low distortion power factor and potentially serious phase and neutral loading concerns.
A typical switched-mode power supply first generates a DC bus voltage using a bridge rectifier or similar circuit. The usable output voltage is then derived from this DC bus by switching and retransformation. The problem with this configuration is that the rectifier is a non-linear device, and so the input current is highly non-linear. That means that the input current has energy at harmonics of the frequency of the voltage. In an alternating current (AC) electrical system, the energy (related to both current and voltage) that is not attenuated by the load flows onto the return side of the circuit that is joined to the neutral from the street transformer. As the AC cycles, the distorted energy flows back through the load causing the load to be less efficient and consume more power. Attempts to solve this problem have been either very complex hot side systems or token systems that lack the proper electrical characteristics to properly function.